WASHINGTON — More than 700 people have left the Environmental Protection Agency since President Trump took office, a wave of departures that puts the administration nearly a quarter of the way toward its goal of shrinking the agency to levels last seen during the Reagan administration.

Of the employees who have quit, retired or taken a buyout package since the beginning of the year, more than 200 are scientists. An additional 96 are environmental protection specialists, a broad category that includes scientists as well as others experienced in investigating and analyzing pollution levels. Nine department directors have departed the agency as well as dozens of attorneys and program managers. Most of the employees who have left are not being replaced.

The departures reflect poor morale and a sense of grievance at the agency, which has been criticized by President Trump and top Republicans in Congress as bloated and guilty of regulatory overreach. That unease is likely to deepen following revelations that Republican campaign operatives were using the Freedom of Information Act to request copies of emails from E.P.A. officials suspected of opposing Mr. Trump and his agenda.

The cuts deepen a downward trend at the agency that began under the Obama administration in response to Republican-led budget constraints that left the agency with about 15,000 employees at the end of his term. The reductions have accelerated under President Trump, who campaigned on a promise to dramatically scale back the E.P.A., leaving only what he called “little tidbits” in place. Current and former employees say unlike during the Obama years, the agency has no plans to replace workers, and they expect deeper cuts to come.

“The reason E.P.A. went down to 15,000 employees under Obama is because of pressure from Republicans. This is the effort of the Republicans under the Obama administration on steroids,” said John J. O’Grady, president of American Federation of Government Employees Council 238, a union representing E.P.A. employees.

ProPublica and The New York Times analyzed the comings and goings from the E.P.A. through the end of September, the latest data that has been compiled, obtained under the Freedom of Information Act. The figures and interviews with current and former E.P.A. officials show the administration is well on its way to achieving its goal of cutting 3,200 positions from the E.P.A., about 20 percent of the agency’s work force.

Jahan Wilcox, a spokesman for the E.P.A., said the agency was running more efficiently. “With only 10 months on the job, Administrator Pruitt is unequivocally doing more with less to hold polluters accountable and to protect our environment,” he said.